intel/compiler: Clear accumulator register before EOT

intel/compiler: Clear accumulator register before EOT

v2: (Francisco Jerez)
- Drop vec4 changes.
- Handle explicit acc0 operand and implicit one.
- Make sure instruction is SIMD16, prediction is off and default mask
  control set to true.

v3: (Francisco Jerez)
- Clear accumulator only when it's written.
- Use BRW_MASK_DISABLE instead of true.
- Use correct width for brw_acc_reg().
- Fix last_inst_offset.

v4: (Francisco Jerez)
- Don't check for last instruction for accummulator write.

Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3376>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3376>